A heterogeneous catalytic process is a chemical reaction that takes place between gaseous and/or liquid reactants and a solid catalyst, where a catalyst is defined as something that accelerates a chemical reaction without being ultimately changed. In reactor configurations including, but not limited to, fixed bed, fluidized bed and slurry phase reactors, it is desirable to optimize the extent of contact between the reactants and the solid catalyst while addressing other important issues such as heat transfer, mass transfer, product removal and gas recycling. With respect to supported catalysts, control of the morphological properties of the support, such as surface area, pore volume, pore size and concentration of the pores per unit volume of support material is desirable because these properties can affect the course of the reaction and the products obtained therefrom. In particular, such properties tend to influence the nature and concentration of active catalytic sites, the diffusion of the reactants to the active catalyst site, the diffusion of products from the active sites, and the useful lifetime of the catalyst. In addition, the support and its dimensions influence the mechanical strength, density and reactor packing characteristics, control of which are desirable.
Alumina is a typical catalyst support material and can exist as any one or combination of crystallographic phases, or polymorphs, commonly known as transition aluminas. Transition aluminas are ubiquitous as supports and/or catalysts for many heterogeneous catalytic processes. The synthesis of the transition aluminas typically begins with the hydroxides or oxyhydroxides of aluminum, both of which are effectively hydrates of alumina. Examples of such include the naturally-occurring or synthetic aluminum hydroxides, such as aluminum trihydroxides (gibbsite, bayerite, and nordstrandite) or monohydroxides (boehmite or diaspore). Progressive dehydration and accompanying lattice rearrangement in the series of transition aluminas leads to increasingly stable and ordered materials, and culminating in all cases with alpha alumina, which is a material unsuitable for catalytic applications and that requires high surface areas. Alpha alumina is typically obtained at calcination temperatures in excess of 1,000° C. There exist preparative processes within the art that can provide for transition aluminas having any number of distinct combinations of desirable properties, e.g., particle size, surface area, pore volume and average pore diameter. Some catalytic processes within the art for which transition aluminas are employed as catalyst and/or catalyst supports subject the transition alumina to conditions of high temperature, high pressure and high water vapor pressure.
Catalytic reactions that produce water vapor of high temperature and high partial pressure create an environment that challenges the hydrothermal stability of transition alumina supports, with the supports being prone to degradation, fragmentation, or other processes that compromise the ability to effectively support catalytic metals. Finding or preparing transition alumina of sufficient hydrothermal stability for use in protracted steam-producing reactor runs remains an important problem in the art. For purposes of the present discussion, hydrothermal stability is defined as the property of resisting morphological and/or structural change in the face of elevated heat and water vapor pressure.
The Fischer-Tropsch process (also called the Fischer-Tropsch reaction or Fischer-Tropsch synthesis) is an example of a process that can generate water vapor of high partial pressure at high temperatures. The Fischer-Tropsch process comprises contacting a feed stream comprising carbon monoxide and hydrogen gases, known as synthesis gas or syngas, with a catalyst at conditions of elevated pressure and temperature to produce mixtures of hydrocarbons and by-products comprising water and carbon dioxide. Syngas can be made from the gasification of coal or, alternatively, methane found in natural gas reserves by partial oxidation with an oxygen source or by reaction with steam (steam reforming). Natural gas is typically stranded gas found with oil deposits during drilling operations. Such stranded gas presents a problem in those areas where there is no close market for this commodity because transportation of gases as compared to liquids is costly and impractical. As a result, on-site conversion of gaseous resources to easily transportable liquids represents a large potential gain in revenue. The Fischer-Tropsch process is one use of syngas and as such presents an attractive market for gas to liquids technology. It has long been recognized that syngas can be converted to liquid hydrocarbons by the catalytic hydrogenation of carbon monoxide. The general chemistry of the Fischer-Tropsch reactions are as follows:n CO+(2n+1)H2→CnH2n+2+nH2O  (1)CO+H2O→CO2+H2  (2)A competing reaction is often the water-gas shift reaction, equation (2), in which carbon monoxide is consumed in a reaction with water generated from equation (1), above, to form carbon dioxide (CO2) and hydrogen (H2). The catalytic metal used can influence the nature and composition of the mixture of products and by-products formed. For example, it is well known that iron-based Fischer-Tropsch catalysts have high water gas shift activity while cobalt-based Fischer-Tropsch catalysts have a much lower water gas shift activity.
Catalysts for the Fischer-Tropsch process typically comprise a metal selected from the group comprising cobalt, iron, ruthenium, or other Group VIIIA (according to the Previous IUPAC Form of the Periodic Table of the Elements as illustrated in, for example, the CRC Handbook of Chemistry and Physics, 82nd Edition, 2001-2002, which will serve as the standard herein and throughout for all references to element group numbers in this application) metals; optionally, a cocatalyst selected from the group consisting of copper, thorium, zirconium, rhenium or titanium; and, optionally, a promoter selected from the group consisting of the alkali metals, the alkaline earths, the lanthanides, Group IIIB, IVB, VB, VIB and VIIB metals; and may be supported or unsupported. The current practice with respect to supported catalysts is to use porous, inorganic refractory oxides as the carrier. γ-Al2O3 is an example of such a carrier.
Fischer-Tropsch reactors utilizing a cobalt-based catalyst can generate significant amounts of water due to the relatively low water gas shift activity of cobalt catalysts. Under typical reactor conditions, e.g., temperatures in excess of 200° C. and pressures in excess of 20 bar, the water produced in these reactions can reach partial pressures in excess of 5 bar. Under these conditions, catalyst support particles, such as those comprising γ-Al2O3 for example, can degrade and disintegrate, causing cobalt to dislodge from the support particles and permitting for the appearance of cobalt fines in the product stream. The formation of subparticles that are in the submicron range in a product stream has multiple undesirable repercussions: 1) purification and complete removal of subparticles from the product stream tends to become quite difficult; 2) a reduced lifetime of the catalyst; 3) regeneration of recovered cobalt catalyst tends to be severely hindered; and 4) the loss of costly cobalt metal can represent a significant loss of revenue.
Other industrial processes that involve steam and consequently require catalyst supports stable to high-temperature and high-pressure steam include steam reforming, water gas shift reaction and catalytic conversion for emission control in automobiles.
Thus, there have been attempts to address the general problem of making catalyst supports that do not degrade at elevated temperatures with concomitant loss of high surface area. For example, U.S. Pat. No. 5,837,634 discloses a process for preparing a stabilized alumina that exhibits an enhanced resistance to structural degradation at high temperatures, e.g., greater than about 1,000° C. The process comprises aging an admixture of a precursor boehmite alumina and an effective amount of a stabilizer such as a water-soluble salt of a polyvalent metal at a pH of from about 3 to about 9 and at a temperature greater than about 70° C. to convert the greater portion of the alumina to a colloidal sol, wherein the colloidal sol is recovered and calcined to produce a stabilized alumina. Surface areas (m2/g) were measured on these stabilized alumina powders after calcination for 3 hours at 1,200° C. and show that the addition of stabilizers results in the persistence of surface areas in about the 10 m2/g to 60 m2/g range.
It will be apparent to one of ordinary skill in the art that the calcination conditions employed in the '634 patent will most likely provide an alpha alumina, which is a polymorph of alumina that is not suitable for some catalytic applications.
Similarly, U.S. Pat. No. 6,262,132 B1 provides a method for reducing catalyst attrition losses in hydrocarbon synthesis processes conducted in high agitation reaction systems, in which the phrase “high agitation reaction systems” refers to slurry bubble column reactor systems and to other reaction systems wherein catalyst attrition losses resulting from fragmentation, abrasion, and other similar or related mechanisms at least approach the attrition losses experienced in slurry bubble column systems. It is disclosed that, in one aspect of the method for producing an attrition-resistant catalyst support, the catalyst support is gamma alumina including an amount of titanium effective for increasing the attrition resistance of the catalyst.
U.S. Pat. No. 6,303,531 B1 relates to hydrothermally stable, high pore volume aluminum oxide/swellable clay composites and methods for their preparation and use. The patent is based on the teachings that when active alumina is dispersed and subjected to a rehydration process in the presence of controlled amounts of a dispersed swellable clay the resulting composite particles exhibit and maintain the properties of high surface area and hydrothermal stability, wherein the properties are retained when catalytically active metal components are impregnated before or after the shaping of extrudates. It is also disclosed that the hydrothermal stability of the composite particles could be further improved by the incorporation of silicate salts therein.
Pore size and mechanical strength in γ-Al2O3 have been influenced by low temperature hydrothermal treatment of γ-Al2O3. As disclosed in Preparation of Catalysts V, 1991, page 155-163, wherein gamma alumina in the form of 1.5 mm extrudates was subjected to hydrothermal treatment in an autoclave in the presence of water vapor, it was found that the crushing strength was observed to increase progressively with increasing duration of heating for γ-Al2O3 hydrothermally treated at 150° C., with a considerable increase (about 65%) in the volume of 100-250 Å diameter pores. At higher temperatures, a reverse trend is noticed. X-ray diffraction analysis of the hydrothermally treated samples showed no peaks corresponding to other phases of alumina.
The problem of contamination of a Fischer-Tropsch product with catalyst ultra fines has been addressed by introducing to an untreated catalyst support a modifying component that is capable of suppressing the solubility of the catalyst support in acidic or neutral aqueous solution. The ultimate effect is that of preventing the formation of loosely bound hydrotalcite-like structures upon which the active catalytic cobalt metal can precipitate and subsequently become dislodged during extended Fischer-Tropsch reactor runs. International Application No. WO 99/42214 discloses that such catalysts have hitherto been produced by slurry impregnation of an alumina support with cobalt nitrate in acidic to neutral solution, a medium in which the alumina is partially soluble. Upon dissolution, the cobalt and aluminum ions can co-precipitate as hydrotalcite-like structures, e.g. Co6Al2CO3(OH)16.4H2O, that are physically adsorbed and loosely bonded to the original alumina surface. Commercialization of the slurry phase Fischer-Tropsch process reveals a serious problem that can arise when such catalysts using the known untreated alumina supported cobalt catalyst are used as the wax product as they could contain relatively high amounts of attrided catalyst. Evidently, during slurry impregnation of an untreated alumina support, cobalt nitrate will deposit on the loosely bonded hydrotalcite-like structures. The cobalt on loosely bonded hydrotalcite-like structures can dislodge during extended runs and contaminate the wax product with cobalt rich ultra fines. Attempts to solve or at least alleviate this problem have included protecting the alumina support during aqueous impregnation by improving the inertness of the alumina surface.
U.S. Pat. No.6,224,846 B1 discloses a process for making a modified boehmite alumina comprising reacting at elevated temperatures a boehmite alumina with an alkyl or aryl mono- or disulfonic acid derivative as the acid or its salt to produce a reaction mixture containing a modified boehmite alumina, with the modified boehmite alumina being recovered from the reaction mixture.
Many in the art have attempted to solve the general problems of catalyst attrition and hydrothermal stability in catalysts. However, creating a catalyst on a stabilized transition alumina support that possesses high hydrothermal stability and low attrition resistance remains a problem. In particular, creating catalysts suitable for use in Fischer-Tropsch reactors, which produce substantial quantities of water vapor at high partial pressure, remains a need within the art. Further needs include providing a stabilized transition alumina having high hydrothermal stability.